Post-metamorphic development of Early Cretaceous frogs as a tool for taxonomic comparisons

ABSTRACT

Specimens of a single but currently undetermined species of the anuran genus Liaobatrachus from the Lower Cretaceous of northeast China, including four articulated skeletons preserved on a single block of sediment plus two skulls and several additional adult skeletons on other slabs, illustrate post-metamorphic developmental stages from early juvenile to fully grown adult. The relative ages of the individuals were estimated based on body size combined with degree of ossification. Post-metamorphic developmental data not only provide a basis for comparisons with other fossil and recent anuran taxa, but also help to identify both age-independent (i.e., size-independent) characters that can be used for taxonomic comparisons among individuals of various sizes (i.e., ages) and age-dependent (i.e., size-dependent, ontogenetically variable) characters that can only be used for comparisons across corresponding developmental stages. Our study revealed that the following characters are independent of size in Liaobatrachus: frontoparietals with a fontanelle between their frontal portions; quadratojugal present; vertebral centra amphicoelous and pierced by a notochordal canal; and free (not ankylosed) ribs attached to diapophyses of second to fourth vertebrae. The tibiofibula/femur and presacral vertebral column/snout-vent length ratios were also found to be size independent. In contrast, ossification of the carpus, distal tarsals, and pubis appears to be ontogenetically variable (i.e., they vary with age), and this is presumably also true of fusion between the tibiale and fibulare. These characters should only be used for comparisons among individuals at a common developmental stage.

ACKNOWLEDGMENTS

This project was supported by the Sino-Czech academic exchange program. Z.R. was a visiting professor at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Academy of Sciences, Beijing, while completing this project; he was also partly supported by the Institute of Geology, Academy of Sciences of the Czech Republic, Prague (Institutional project AV0Z30130516). X.-L. Wang (IVPP) kindly discussed relative age of the Lujiatun Bed with L.D. Y.-F. Guo (IVPP) prepared the specimens studied in this project. We thank A. Henrici, A. M. Báez, C. Sullivan, and J. Anderson for the English editing, and A. Henrici and A. M. Báez for their review and constructive suggestions. The project was partly funded by National Natural Science Foundation of China (grant number 41072015 to Y.W. for preparation of the specimens).

Handling editor: Jason Anderson